In the context of the present description, the term "deck" is used for any type of superstructure for a platform installed at sea. The deck conventionally includes a plurality of vertical tubular legs made of steel or of concrete or partially of steel and partially of concrete, which legs are placed on and fixed to a support structure. The term "support structure" designates any type of infrastructure, sometimes called a "jacket" in this technical field, and designed to support the deck of the offshore platform. In operation, the support structure may be entirely or partially immersed, and it may or may not stand on the seabed. The support structure usually includes a number of vertical or substantially vertical tubular members and/or piles corresponding to the number of legs of the deck. The said tubular members of the support structure, also called "legs", are referred to as "vertical members" in the present description for simplification purposes, given that these members may be genuinely vertical or may slope somewhat relative to the vertical or they may be vertical in part and sloping relative to the vertical in part. The vertical members and/or piles may be made of metal or of concrete or partially of metal and partially of concrete. In addition, in the context of the present description, the term "barge" is used to designate any ballastable floating vehicle capable of transporting the deck of an offshore platform.
The deck and the support structure of an offshore platform are usually prefabricated separately on land or in a dry dock or in a graving-dock, and they are then convoyed or towed separately to a site at sea where they are subsequently assembled together. The assembly site may be the utilization site of the platform, or any other site chosen for sufficient depth of water and sea conditions that are relatively calm.
Several techniques have already been proposed for installing the deck of an offshore platform on a support structure at sea. For example, one known technique is described in the article entitled "Offshore Installation of an Integrated Deck Onto a Preinstalled Jacket", by G. J. White et al., OTC 5 260, Offshore Technology Conference, 18th Annual Conference at Houston, Tex., May 5-8, 1986. In that known technique each leg of the deck contains a hydraulic cylinder and plunger piston assembly, and each vertical member or pile of the support structure includes at its top end a receiver portion suitable for receiving the bottom end of the plunger piston associated the corresponding leg of the deck. That known method comprises the following operations:
a) bringing a barge between the vertical members or piles of the support structure, with the deck supported on the barge by a plurality of retractable supports; PA1 b) positioning and holding the barge in such a manner that the legs of the deck are and remain substantially in alignment with the corresponding vertical members or piles of the support structure; PA1 c) lowering the plunger pistons until their bottom ends come into abutment against the receiver portions of the corresponding members or piles of the support structure; PA1 d) ballasting the barge to lower it and transfer the load of the deck to the support structure; PA1 e) subsequently retracting the supports situated between the deck and the barge so that the deck is supported solely by the support structure; PA1 f) making rigid connections between the legs of the deck and the vertical members or piles of the support structure; and PA1 g) evacuating the barge from between said vertical members or piles.
Operations f) and g) may be performed in the above-specified order, in the reverse order, or simultaneously.
In the open sea, that known technique presently suffers from limits that are due in particular to the relative movements between the deck and the support structure which are caused by the swell and which can give rise to unacceptable stresses or impacts in the assembly constituted by the deck, the barge, and the support structure. In particular, during the operation of ballasting the barge, while said barge is still supporting at least a part of the weight of the deck, and in spite of the presence of shock absorbing devices, the above-mentioned relative movements can cause impacts to occur both between the plunger pistons and the corresponding receiver portions of the support structure and also between the bottom ends of the deck legs and the top ends of the vertical members or piles of the support structure. In that known technique, impacts between the bottom ends of the deck legs and the top ends of the vertical members or piles of the support structure are practically inevitable as the ballasting operation approaches its end and the bottom ends of the deck legs come close to the top ends of the vertical members or piles of the support structure. Given the very large masses involved (the weight of the deck may be several thousands or several tens of thousands of tons), the above-mentioned impacts give rise to battering or deformation of the elements that strike one another. The battering or the deformation can subsequently make it very difficult or even impossible to connect the deck legs to the support structure.
Furthermore, in that known technique, the receiver portion provided at the top end of each vertical member or pile of the support structure comprises a guide tube whose top end is flared to receive and center the plunger piston associated with the corresponding deck leg. Once the plunger piston has penetrated into the guide tube, the horizontal components of the movement of the deck (surge) induced by the swell generate horizontal forces. These make it necessary to provide shock absorber devices operating in compression between the guide tube and the outer cylindrical wall of the vertical member or pile of the support structure. In spite of the presence of such shock absorbers, the support structure and, by reaction, the structure of the deck, can be subjected to unacceptable stresses that may harm the integrity of these structures.